Mass spectrometry



Dec. 28, 1948. R. v..1 ANGMU|R MASS SPECTROME'RY 4 Sheets-Sheet 1 Filed April 2O,l 1943 lNvENToR Aad 'ATTORNEYS De.28,194s. R.V.'LANGMU|R 2,457,162

MASS SBECTROMETRY Filed April 20, 1943 4 Sheets-Sheet 2 INVENTOR n f Klang/27am ATTORN EY5 Dec. z8, 1948. n. v. LANGMUI'R V 2,457,162

. MASS SPECTROMETEY y Filed April 2o, .1943 l 4 sheets-sheet s INVENTQR Wawy/wwf 77 ATTORNEYS Dec. 28, 1948. R. v. LANGMUIR 2,457,162

' MAss SPECTROMETRY Filed April 20, 1943 l 4 Sheds-Sheet 4 v v I /cw eraf/mmv@ 1005465 INVENTOR oew wggwazb* A C. V0 TAGE 4MM/7006 Patented Dec. 28, 1948 MASS SPECTROMETRY Robert V. Langmuir, Schenectady, N. Y.,Yassignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application April 2o, 1943, serial No. 483,145 1o claims. (o1. 25o-41.9)

This invention is concerned with mass spectrometry and contemplates improvements in mass spectrometry involving the production of a pulsating ion current which can be ampliiied with A. C. instruments.

In mass spectrometry, a gas sample is bombarded by moving electrons to produce ions of various substances present in the sample, and the ions thus formed are separated into various compounds having diiierent mass-to-charge ratios by subjecting the ions to the influence of electric or magnetic fields or both. The individual components are then directed upon an ion collector and discharged, and the intensity of the resulting ion current is measured. Thus, the several components may be caused to fall successively upon the collector by varying the electric or magnetic eld, or by moving the collector successively into the respective paths of the several /components.

In a method of mass spectrometry heretofore proposed, the several components having different mass-to-charge ratios are formed of positive ions. When such components are directed to the ion collector and there discharged, a unidirectional current is set up in a circuit connected to the ion collector and the intensity of this current is measured following amplication b-y a direct current amplier. However, this method (especially when the samples of gas to be treated in the mass spectrometer are small) is subject to certain inherent disadvantages arising principally out of the nature of direct current ampliilers. The speed with which a mass spectrum may be measured With a given degree of accuracy depends upon the time constant of the input circuit of the D. C. amplifier. This is necessarily long because of the large input resistance required to detect the small currents available at the collector. The time constant is the product of the input resistance times the input capacitance and may be several seconds or even of theorder of minutes. Thus, employing a direct current amplier, the rate at which a small sample of gas can be analyzed in a mass spectrometer is relatively slow and the capacity of the instrument in terms of useful Work is limited.

As disclosed in my co-pending application Serial No. 294,346 filed September 11, 1939, now United States Patent No. 2,370,673 (of which the instant application is aA continuation-inpart), I have discovered that the speed of analysis can be increased markedly by producing a pulsating ion beam, discharging this beam at the ion collector and amplifying the resulting alternatf ing ion current with an alternating current amplifier. In accordance With my invention, an ion beam is formed in a mass spectrometer by bombardment of a gas with the electrons and the intensity of this ion beam is Varied in a predetermined manner. Thus, the ion beam may be caused to pulsate and the pulsating ion beam, upon discharge at an ion collector, produces an oscillating potential which is amplified with an A. C. typeamplier and recorded or measured. For example, a plurality of separate ion beams of different mass-to-charge ratios may be generated or controlled in such fashion that the intensity of each beam varies in a sinusoidal manner. The pulsating ion beams thus formed are caused to fall successively at the collector to produce a mass spectrum. Preferably, the frequency of the ionbeams should be smaller than the number of ions falling on the collector per second. Otherwise, statistical variations in the rate at which the ions fall on the collector may mask the pulsations in that rate, and it is these pulsations Which should be detected.

The required pulsating ion beam may be formed in several ways, but in all cases the ions of a given mass-to-charge ratio strike the collector and there discharge at a rate which oscillates, pulsates or varies in a regular manner to produce a corresponding pulsating or oscillating electric current which may be amplified with an alternating current amplier. The required oscillation 'may be brought about (1) b-y varying the intensity of the beam of ionizing particles (such as electrons) with which the gas in the mass spectrometer is bombarded; (2) by varying the velocity (or energy) of these ionizing particles (electrons) and (3) by varying the forces acting upon the ions during the separation of ion components having different mass-to-charge ratios, i; e. by varying the eld forces which control the deflection of the ions during separation into the components. f

Thus, in a mass spectrometer having meansfor bombarding a gas sample with ionizing particles to produce ions of substances present in the sample, means for separating the ions thus produced according to their mass-to-charge ratios into a plurality of ion beams, means for pulsating the ion beams, and means for collecting ions of a beam to produce an alternating or pulsating current, my inventionA contemplates the combination which comprises an electrode disposed adjacent the path of the electron stream, means for supplying a potential to the electrode, and means for varying the potential at said electrode in a pair of plates (one of which has an aperture through which the electrons pass into the analya'- ing portion of the apparatus) Another` manner for causing the ion beam to pulsate is to supply a pulsating potential to a solenoidsurroundiiig the analyzing portion of the apparatus, 'whreb'y'i the radius of curvature of the ion beam is caused to vary in a pulsating manner. Thei'on. beam may also be caused to pulsate (a) by causing.

the electron beam to pulsate at its sou'1'r'c':`e",foi"V example a i'ilament, by Varying the temperature at that point and hence the rate of electron emis- `sion or (b) by means of a shutter whchloriodicallyis interposed in the path of 'the ionizing particles.

These and other lfeatures of my invention will be more` thoroughly understood in the llight of the following detailed descriptionY taken in con#- junction with the accompanying 'gures, in which:

' Fig. 1 is a cross-section of one form of mass spectrometer constructed in accordance with my invention (provided with anvanplier, arecorder and apotential source, all shown schematically) and adapted to bring aboutpulsation of the ion beam through modulating the beam of electrons employed to bombard the gas from which the ionsare formed, the electron beam being modulated by varying the accelerating voltage operating upon the electronbeam;

Fig. 2 is a longitudinal view, partly in section, rtaken through the mass spectrometer ofr Fig. 1 along the line 2 2;

Figs. 3, 4, 5, 6, 7 and 8 are longitudinal views through modified forms of the spectrometer of Figs. 1 and 2 and illustrate a variety of Ways Vin which the pulsating ion beams may be formed, like parts being designated by thefsame 'reference characters employed in Figs. 1 and 2;

Fig. 9 is a graph showing the relationship of ion accelerating voltage to the voltage in the resistance 2e of the collector circuit for the types of mass Vspectrometer illustrated iny Figs. 2; 3 and '7; i. e. in Fig, 9 anA. C. voltage component is superimposedon the D. C. voltage that vwould be Ypresent across resistor 36A if the baiii intensity did not pulsate, the A. C. component being V'proportional to the D; C. voltage Vat each point;

Fig. 10 illustrates the relationship ofthe ion accelerating voltage to the amplitude of the A.

' C-Z voltage 'in the output of the amplier circuit in the spectrometer illustrated in Figs. 2; 3 and '7; and Y Fig. 1l illustrates the relationship of ion accelerating voltage to the A. C. voltage amplitude in the output of the amplifier circuit in the apparatus of Figs. 4, 5, 6 and 8.

Referring to the drawingsandparticularly to Figs. 1 andY 2, it will be observed that the mass spectrometer has an envelope 2l which may evacuated through vacuum lines 22, 23 connected to the envelope and maintained at. low pressure by one or more' VacuumV pumps (not shown) connected to the vacuum lines.

A capillary tube 2li is connected to the en# sating manner.

4 velope and through it a sample oi gas to be analyzed is admitted. Within the envelope adjacent the entrance of the capillary there is disposed a pusher plate 25 and a plate 25 having a slit 2l. The plates are disposed respectively on either side of the entrance of the capillary tube so that a gas sample entering the envelope flows into `the space between the plates and is there bombarded b'y a unidirectional electron stream, the strength of which is varied or modulated in a pul- The bombardment of the gas molecules results in the formation of gaseous ions Yin the space between the plates, the amount of 4lio" s formed unit time being correspondingly variedin al pulsating manner.

A small negative voltage (relative to the voltage-on the pusher plate 25) is established on the plate 2 6, In consequence, positive gaseous ions formed in the space are drawn toward the plate 26. Some of these ions pass through the slit 2l in this plate. A semi'ecylindri'cal case 26 is disposed Within the envelope adjacent the plate 25. A 'dat plate- 29 forming one side of the case is dis posed substantially parallel tothe plate 255 and is provided with a second slit' 3! which matches the' slit in plate 26. A high accelerating poten tialV is maintained between' the plate 2t and the case 28.1 This potential provides an electric accelerating eld which causes the positive ions that pass through the rs't slit 2l to be highly accelerated towardsthe case. Some of these accelerated ions enter the case 22 through the second slit 30. l

A solenoid 3|l is disposed around the envelope and produces al magnetic eldtherein, so that the ions entering. the case through the second slit at high velocity follow curved paths within the v Dueto the combined action of the aforementioned accelerating field maintained between the plate 26r and the case and the magnetic eld prof duced by the solenoid, the ions entering the case are separated into ionic beams of different massto-charge ratios. -Each beam follows a semir circular path; `the radius of which is determined bythe strength of the `electric accelerating field, the strength ofthe magnetic iield and the mass to-charge ratio ofthe ions in the particular beam.

. Thus, each beam in effect, originates at the slit 30 and follows itsown curved path to focus on the flat plate 29 opposite the slit 36;

A third slit 32 is provided in the flat plate of the casein the region where the beams tend to focus. Ions having a pre-determined'mass to-charge ratio fol-low a substantially semi-cirn cular Path 33 from Vthe second slit 3l! to the third slit 32, pass through the third slit and fall upon anlion collector 34 which isprotected by a shield 34A having therein an aperture 35 matching the third slit.

N The ions. discharge on thev ioncollector and producethr'ougha resistorSA of a linear ampljiierv 36A connected'to they ion collector a pulsating electriccurrent that corresponds in frequency andintensity to-the collected ion current.

Thefvoltage which results in the resistor et is amplified with anA.' C. ampliiier combination discussed in detail hereinafter and the amplied current is recorded by a galvanometer 37, so thatthe intensity-of the ion beam being discharged at any instant is indicated.

The several ion beams ofr diierent mass-to- 'chargeratio are caused to pass successively tric field between the plate 26 and the case, this being accomplished by moving a slider 38 on a potentiometer 39. In short, the separate beams existing within the case are successively brought to a focus at the slit 32, so that the intensities of the respective beams are measured successively.

To consider one embodiment of my invention for varying the ion current in a pulsating manner, reference should be made to Fig. 2. As shown in this figure, the electrons employed to bombard a gas sample in the space between the plates 25, 26, originate at an ionizing source represented by a filament 40, an anode structure 4| and a control electrode 42 with associated batteries 42A,-

46 and 6|. The iilarnent, the anode structure and the control electrode are disposed within an extension 2|A of the envelope at one end thereof. A D. C. potential difference between the anode and the control electrode is maintained by a battery 6i, which also supplies current to the plate 25.

The control electrode is disposed between the filament and the anode structure and has an aperture therein in line with two other apertures in the anode structure so that the electron beam is directed into the space between the plat-e 25, 26 approximately at right angles to the capillary tube through which the gas sample is admitted0 The electrode and the anode structure preferably are maintained at positive' potentials with respect to the filament.

The electrode 42 is connected to a voltage generator 43 which supplies an alternating potential and thus brings about a periodic variation in the potential of the electrode with respect to the filament. This alternating potential at the electrode controls the intensity of the electron beam originating at the iilament and streaming through the apertures in the control electrode and the anode structure to enter the ionization region between the plates 25, 26 where the electrons bombard and ionize gas introduced from the capillary tube.

Some of the electrons of the beam or stream i may pass all the way between the plates to be picked up by an electron collector or cage 44 disposed in line with the apertures in the control electrode and the anode structure. The Vagrant electrons thus picked up flow to ground through a battery 45 connected to the cage.

The variable voltage at the control electrode varies the intensity of the electron beam entering the ionization region, but the velocity of these electrons is substantially constant and is determined by the potential difference between the iilament andthe anode.

A battery 46 supplies potential between the iilament and the control electrode and the voltage of this battery is in series with the variable voltage supplied from the A. C. generator 43. If the voltage of the battery 46 exceeds the amplitude of the variable voltage (for example a sinusoidal voltage) supplied by the generator, the electron beam will vary in intensity but will never be shut oi completely.

It will be apparent that the concentration of ions in the space between plates 25, 26 will be substantially proportional to the intensity of the electron beam if the velocity of the electrons is constant.

The frequency of the electron beam modulation, which is controlled by the frequency of the A. C. potential supplied by the A. C. generator, preferably is less than the average number of ions falling on the ion collector per second, so that statistical variations in the rate at which the ions reach the collector will not obscure the pulsations that are to be measured. In this manner, the intensity of the ion current varies in an easily controllable manner. However, some of the advantages of my invention may be retained if the ion beam is modulated at a frequency higher than the average number of ions falling on the collector per second.

To facilitate amplification of the output of the mass spectrometer, it is preferable to produce pulsating ion currents that vary in a sinusoidal manner, this being accomplished by employing an electron beam that pulsatesin this fashion.

The alternating portion of the ion current appearing at the collector 34 is amplified in a circuit including the iirst linear amplification stage 36, a second linear amplification stage 41, a narrow band pass iilter 46, a logarithmic amplifier 49 and indicated at the recording galvanometer 31. The two amplification stages are coupled through the coupling and blocking condenser 41A, which lters out the D. C. component of the signal appearing in the output of the first stage.

The amplifier stage has, as indicated above. a linear characteristic and comprises one or more tubes. Thus, the iirst amplifier stage may comprise an amplifier tube 50 of the pentode type provided, as shown in Fig. 1, with a control grid 5l, a screen grid 52 and a suppressor grid 53 together with a cathode 54 and an anode 55. Suitable resistors, including the resistor 36A and a second resistor 56 are connected to suitable batteries'for maintaining the control grid and the anode respectively at correct operating potentials. The input capacity between the control grid and the cathode is represented by the dotted structure 51.

The alternating portion of the signal Voltage appearing between the screen grid 52 and the cathode 54 may be calculated according to the following formula where 1; is the alternating portion of the ion current falling on the collector 34 and C is the input capacitance 51 of the tube 50. As indicated above, the corresponding output voltage appearing at the anode o-f the rst amplifier stage 36 is further ampliiied by the amplifier 41, passed through the narrow band pass filter 48, amplified further by the logarithmic ampliiier 49 and impressed on the recording galvanometer 31. The band pass lter preferably is designed to pass a very narrow band of vfrequencies in the region of For this purpose a resonant circuit having a very high Q may be employed. For example, a Q obtained from a resonant circuit utilizing electromeechanical units such as magneto-striction oscillators or tuning forks is desirable. The use of a narrow band pass filter or a sharp resonant circuit permits an increase in the signal-to-noise ratio of the circuit so that smaller ion currents than would otherwise be possible can be detected. This will be apparent from the following:

Due t0 thermal agitation within the resistor 36A, noise will be impressed on the controlgrid 5I and ampliiied by the system. If only a' small entame EN df approx.

where 'I' is temperature in degrees Kelvin, K is Boltzmans gas constant, and df is the effective band width of filter 48.

Hence the signal-to-nose ratio in the output is Inasrnuch as the current i to be measured may be as low as l-17 ampere, I prefer to utilize as large a resistance 36A as possible, of the order of 101 or 1014 ohms, and the narrow band pass filter 48, and inthis manner achieve high ultimate sensitivity and provide high signal-to-nolse ratio.

Following the filter 48, I prefer to utilize an amplifier the loutput of which is proportional to the logarithm of the input, thus making possible the recording of a wide range of ion intensities on a recording medium of limited width. Logarithmic ampliers suitable Yfor this purpose are well known to those skilled in the art.

Inthe form of my invention illustrated'I utilize a common control connection 58 to coordinate the movement of the recording medium within the recorder 37 with the magnitude of the accelerating or analyzing electric field provided between the plate 26 and the case 28 .by the potentiometer 38. In this manner, I am able to produce a continuous record in which one coordinate indicates the mass-to-charge ratio and the other coordinate measures intensity of ion current.` Thus, the control connection is connected Vnot only to the potentiometer and the galvanometer but also to the case 28.

It is to be understood that the recording speed necessary to attain a predetermined .resolving power varies as an inverse function of the Width (df.)- of the band passed by the filter 48, the term resolving power being employed to dene the ability t0 differentiate and accurately measure the relative intensity of neighboring peaks on the nal record obtained.

It is clear that the ultimate sensitivity, resolving power, and recording speed are interrelated with the characteristics of the 'filter 48. In the `preferred form of my invention the recording -speed is made as rapid 'as possible without causing undue lossofsensitiv'ty or resolving power.

In the apparatus of Figs. 1 and 2, it will be apparent that the pulsation or modulationof. the ion current is a result of pulsation or modulation of the electron beam employed t0 produce the ions by bombardment and that this pulsation of the electron beam is brought about by varying the potential of a control electrode disposed between the filament and the anode. The ion beam may also be modulated in other ways. Several of these alternative methods of modulating the ion current are illustrated and described in conjunction with Figs. 3 to 8, inclusive, These figuresv show modifications of the apparatus of Fig. 2, the parts which are dissimilar to those in Fig. 2 being indicated.

To consider the modification illustrate-d in Fig. 3, it will be observed that this apparatus is essentially similar to that of Figs. 1 and 2 except that the alternating current generator 43 is eliminated, there being instead an alternating current source 60 -connected between the filament 40 and the -apertured anode 4i' in series with a direct current battery 6l The alternating current provided from the alternating .source/68 serves to vary the velocity of the electrons passing through the apertured anode lll into the ionizing region between the pusher plate 25 and the plate 2t. The plate 26 is maintained at ground potential and the -pusher plate is` maintained at a positive voltage by the battery 6l.

In this form of my invention, the potentials supplied by the battery 46 preferably are `sufficiently large to provide the electrode i2 with a potential that is adequate to draw off the maximum possible number of ele-ctrons from the filament and project them through the aperture in the control electrode 42 so that an electron beam of constant intensity is provided beyond this point. Under these circumstances, the alternating voltage provided by the A. C. source 60 serves to vary both the rate at which the electrons enter the ionization region and the velocity with which the electrons enter. Both of these effects combine in an additive manner to vary the rate of production of ions in the ionization region, with resultant production of a pulsating ion current. Ion beams of diiierent mass-to-charge ratio are successively focused at the collector 34 (see Fig. 1) and produce .an alterhating current through the resistor 36A, as described hereinbefore in connection with Fig, 2.

, In another embodiment of my invention illustrated by Fig. 4, the alternating current source of Fig. 2 is eliminated and instead, the anode :il is connected to the positive terminal of the battery 6l and a source 64 of alternating Voltage is connected between the anode and pusher plate. Thepusher plate and the plate 26 causes the energy and therefore the momentum with which ions are projected into the case 28 to vary in a corresponding periodic manner. rThe result of this is to bring about a periodic` variation in the radius of the path 33 overwhich ions of a given mass-to-charge ratio travel from the ionization region to the exit slit 32. In these circumstances, the intensity of the ion beam that passes through the exit slit and impinges upon the collector also varies with resultant production of an alternating current through the resistor 36A.

Substantially the same effect as that obtained in the apparatus of Fig. 4 can be obtained as shown in Fig. 5, wherein the alternating current source 3 Iof Fig 2 is eliminated and replaced by an alternating current source connected in series with a battery 66 between the electrode 2t and the ground Sl. Likewise, the effects obtained in the apparatus ofFig. 4 may be obtained as shown in Fig. 6A wherein the alternating potential source 43 of Fig. 2 is eliminated and replaced by an alternating power k'HJ connected between the case 28 and the slider 38 of the potentiometer 39.

Still another method for modulating the electron beam is .illustrated in Fig. 7. As in the pre- --vio'us cases, (Figs. I.3 to 6) the alternating current source 43 of Fig. 2 is eliminated, but is replaced by an alternating -current source'l connected in the lament circuit in series with a direct ycurrent battery 14. The alternating current supplied to the filament causes the temperature of the ulament to alternate in a corresponding manner with 'resultant variation in thermionic emission (of electrons) from the filament.

In the apparatus of Fig. 8, modulation of the ion beam is brought about by varying the magnetic eld provided by the solenoid 3 i Thus, the alternatng voltage 43 of Fig. 2 again isomitted but is replaced by an alternating currentl source '1S connected in series with the direct current source l1 aasaioa that supplies the solenoid. The change in intensity of the magnetic field produces corresponding changes in the radius of the ion beam path 33, so that each ion beam is swept back and forth across the exit slit 32 for a given setting of the potentiometer 39. Naturally enough, the particular ion beam to be investigated can be `selected, b'y choosing an appropriate setting for this potentiometer.

In all of the foregoing cases, i. e. in the alternatives illustrated in Figs. 1 to 8, an alternating current is produced in the output of the spectrometer.

In the apparatus illustrated by Figs. 2, 3, and 7, the alternating current in the outlet circuit, in resistance 36A is of the same frequency as that generated in the respective alternating voltage sources 43, 6U and 13. In all of these cases, the ion accelerating voltage provided by the potentiometer 39 is varied gradually. As each beam passes by the exit slit, some of the ions impinges upon the plate 29 of the case. The rate at which ions from agiven beam pass through the slit 32 pulsates yor varies periodically, so that the rate of collection of charges from the ions at the collector 3d is likewise modulated.

The manner in which the rate of` collection at the collector St varies in the case of the apparatus oi Figs. 2, 3 and 7 is shown by Fig. 9. The alternating current voltage across the resistance 36A is, of course, proportional to the rate of ion collection, so the voltage across the resistance 38A will likewise vary as illustrated by Fig. 9. However, the capacitance effect of the condenser 47A (Fig. l) blocks any D. C. component of the voltage generated in the plate (cathode) circuit of the tube 52, so that the voltage appearing at the output of the amplifier 36 will be of the alternating current type and will Vary in amplitude as the voltage supplied by the potentiometer 39. For ions of a given mass-to-charge ratio, the voltage amplitude in the output of amplier 36 is a single peak as shown in Fig. 10.

To consider the examples illustrated in Figs. 4, 5, 6 and 8, it should be noted that the intensity of the current in the resistance 36A at the output of the spectrometer is proportional to the absolute value of the derivative of a curve obtained by the foregoing methods, so that a double peak q-q (Fig. 11) is produced for ions of each mass-to-charge ratio detected.

The height of the single peak of Fig. 10 or the height of either part q of the double peak q-q of Fig. 11 is proportional to the concentration of a gas from which ions of the particular mass-to-charge ratio are being produced, the constant of proportionality depending on the individual gas, the mass-to-charge ratio of the ions in question, and naturally upon the constants of the apparatus.

As described hereinbefore sources of square wave voltages may be provided, in which case the intensity of the beam striking the collector 34 is periodically changed between two values for a given setting of the potentiometer. This results in a production of square wave voltages across resistance 36A. The square wave voltages contain numerous sinusoidal components and any desired component may be selected from the square wave and amplified. It is preferable by means of filter 48 to select the strongest component, as determined from a Fourier analysis of the square wave voltage across the resistance.

If the alternating square Wave voltage provided by the source has adequate amplitude, the

10 beam striking the collector may be intermittently cut off completely.

The apparatus of Figs. 3 to 3 may be connected with the same ampliiier circuit as shown in Fig. 1, namely, to a suitably designed band pass filter 48 that makes possible increasing the signal-to-noise ratio. With all of these apparatus a logarithmic, or other variable sensitivity, amplifier can be provided which amplies weak signals more than strong signals, so that signals of widely varying strength may be accurately recorded with a conventional recording galvanometer 31.

I claim:

1. In a mass spectrometer having means for bombarding'a gas sample with an electron stream to produce ions of substances present in the sample, means for separating the ions thus produced into a plurality of ion beams of different massto-charge ratios, and means for collecting ions of a beam, the combination which comprises a pair of plates disposed in the spectrometer in the region where the electron stream bombards the gas sample to form the ions and adjacent the electron stream, one of said plates being provided with an aperture through which the ion beam collected at said collecting means passes, and a source of alternating potential connected between said Iplates for creating an alternating eld therebetween, whereby the ion beam collected at said collecting means is pulsated so as to create an alternating current component at said collecting means.

2. In a mass spectrometer having means for bombardng a gas sample with an electro-n stream to produce ions of substances present in the sample, means for separating the ions thus produced into a plurality of beams of ions of different mass-to-charge ratios, and means for collecting ionsof a beam, the combination which comprises a `pair o plates disposed in the spectrometer and in lspaced relationship to each other adjacent to and on opposite sides of the electron stream in the region wherein the gas sample is bombarded with the stream to'produce the ions,

one of said plates having an aperture therein through which the ion beam passes, and means for producing an alternating potential on the plate having the aperture therein relative to the other plate whereby the ion beam collected at said collecting means is pulsated so as to create an alternating current component at said collecting means.

3..In a mass spectrometer having means for bombardng a gas sample with an electron stream to produce ions of substances present in the sample, means for separating the ions thus produced into a plurality of beams of ions of diiferent mass-to-charge ratios, and means for collecting ions of a beam, the combination which comprises a solenoid surrounding the space in the mass spectrometer in which the ions are separated into ion beams having different mass-tocharge ratios, means for supplying current to the solenoid and means for pulsating the potential of the current supplied to the solenoid, whereby the ion beam collected at the collecting means is pulsated to produce an alternating current component at said collecting means.

4. In a mass spectrometer having means for bombardng a gas sample with an electron stream to produce ions of substances present in the sample, means for separating the ions thus produced into beams of ions of different mass-tocharge ratios, and means for collecting ions of a each other in the region in which the ions are formed and on opposite sides of the electronr stream, one of said plateshaving an orifice therein Vthat matches the orifice in the case, and means for establishing a pulsating potential between the orice plate and the case, whereby the ion beam'collected at said collecting means is pulsated to produce an alternating current component at said collecting means. f

5. Inja mass spectrometer having means vfor bombarding a gas sample with an electron stream to produce ions of substances present in said sample, means for producing field iorces'for separating the ions thus produced into a plurality of ion beams of diierent mass-to-charge ratios, and means for collecting ions o-f a beam, the combination which comprises means for producing an electron stream which traverses the region of the gas sample to'produce said ions of said ion beams, and means for uniformly pulsating a field force which controls the deflection of the ions during said separation into the components Vhaving different mass-to-charge ratios.`

6. In a mass spectrometer having means for bombarding a gas sample with an electron stream to produce ions present in said sample, means for producing eld forces for separating the ions thus produced into a plurality of ions of different mass-to-charge ratio, one of said field forces being an electrical eld employed to propel the ions in the direction of the beams, and means for collecting ions of a beam, the'combination which comprises means for producing an electron stream which traverses the region of the gas sample to produce said ions of said ion beams, and means foruniformly pulsating said electrical'field during said separation of the ions into `components having diierent mass-to-charge ratios.

7. In a mass spectrometer having means for bombarding a gas sample with an electron stream to produce ions of substances present in said sample, means for producing neld forces for separating the ions thus produced into aplurality of ions of diierent mass-to-charge ratios, one of said'fleld forces being an electromagnetic iield employed todeect the beams and means for 'collecting ions of a beam, the combination Ywhich comprises means for producing an electron stream which traverses the region of the gas sample to produce said ions of said ion beams, and means for uniformly pulsating the electromagnetic eld during said separation of the ions 12 into components having diiierent mass-to-charge ratios.

8; In mass spectrometry involving the productionv of ions and the separation of these ions by eld'iorces which control their deection into a plurality of ion beams having different mass-tocharge ratios, the improvement which comprises pulsating a eld force which controls such deflection during the separation so that the beams are oscillated,'and collecting ions of a resulting oscillating beam to produce an alternating cur# ren .9. In mass spectrometry involving the production of ions and the separation of these ions by field forces which 'control vtheir deflection into a plurality of ion beams having diierent massto-charge ratios, the improvement which comprises pulsating an electric held force which controls such deflection during the separation so that the beams are oscillated, and'collecting ions of a .resulting oscillating beam to produce an alternating current.

10. In 'mass spectrometry involving the production of ions and the separation of these ions by field forces which control their deflection into a plurality of ion beams having different mass-to-Charge ratios, the improvement which comprises pulsatingr an electromagnetic eld force which controls such deflection during the separation so that the beams are oscillated, and collecting ions of a resulting oscillating beam to produce an alternating current.

ROBERT V. LANGMUIR.

REFERENCES orrnn .'lhe following references are of record in the file or" this patent: Y

Bainbridge, an article entitled, A Search for Element r87 by Analysis of Positive Rays, by K. T. Bainbridge. Pages 752-762 of Physical Review, vol.;34, Sept. 1, 1929.

Smith et al.,.an.article entitled, A High Sensitivity Mass Spectrograph with an Automatic Recorderfby'l?. T. Smith, W. W. Lozier, L. G. Smith, and W. vBleakney on pages 51 to 55 of Review of .Scientific Instruments, vol. 8, New Series, Feb. 1937.

Tateet al.,y an article entitled A Mass Spectrum Analysis of the Products of Ionization by Electronlmpact in Nitrogen, etc., by J. 'I'. Tate, P. 'I'. Smith, and A. L. Vaughan on pages 525 and 526 of Physical Review, vol. 48, Sept. 15, 1935. f

Dempster,r an article entitled, Ion Sources for Mass Spectrometry, by A. J. Dempster on pages 46 to 49 of Review of Scientific Instruments, vol. '7, New Series, Jan. 1936. 

